1. Field of the Invention
The present invention relates to a holding device for holding an optical element to be ground, such as a lens, a prism or the like.
2. Description of the Related Art
Prior arts which relate to holding devices of this kind are disclosed in Japanese Patent Laid-Open No. 121862/1986 and Japanese Utility Model Publication No. 37627/1987.
In accordance with the art disclosed in Japanese Patent Laid-Open No. 121662/1986, there is provided an apparatus of the type which, as shown in FIG. 21, includes a grinding disc 201 for grinding a lens 203 while rotating in a fluctuating manner and a grinding holder 202 which is held such that its radial plane is perpendicular to the central axis of the grinding disc 201, the lens 203 being ground by the grinding disc 201 as it is pressed against the grinding disc 201 by means of the grinding holder 202. In the construction of the apparatus of this prior art, an elastic member 204 is attached to the end face of the grinding holder 202 which opposes the lens 203, and the grinding holder 202 is fitted onto a holder spindle 205 inserted into a holder shaft 206 so as to elastically apply a pressure to the lens 203 to be ground between the grinding disc 201 and the grinding holder 202. Reference numeral 207 denotes bearings for supporting the holder spindle 205 for rotation about the longitudinal axis thereof, and reference numeral 208 denotes a spindle connected to the grinding disc 201.
The feature of the grinding mechanism having the aforesaid arrangement is that the grinding holder 202 having the elastic member 204 at the end facing the lens 203 is fitted onto the holder spindle 205 inserted into the holder shaft 206.
In accordance with the art disclosed in Japanese Utility Model Laid-Open No. 37627/1987, there is provided a lens holding device for use in an apparatus of the type which, as shown in FIG. 22, includes a grinding disc 210 arranged to rotate about the longitudinal axis of the apparatus as indicated by the arrow shown in a lower portion of the figure and a lens moving rod 211 arranged to travel in the horizontal direction while pressing the surface of the grinding disc 210 with its longitudinal axis always kept normal to the surface of the grinding disc 210. This lens holding device comprises an inverted-cup-shaped member 212 secured to one end of the lens moving rod 211 for rotation about the axis thereof, a lens holder 214 removably fitted into the inverted-cup-shaped member 212 and engaged with the edge of the open end of the inverted-cup-shaped member 212 via an elastic member 213, the lens holder 214 being capable of tilting by slight angles with respect to the inverted-cup-shaped member 212, a recess means 216 formed in the lower end portion of the lens holder 214 which is radially inward of the edge of the open end of the inverted-cup-shaped member 212, the recess means 216 serving to securely receive a lens 215 while reducing a moment resulting from the horizontal travel and acting to rotate the lens 215, a bore 217 formed in the lens holder 214 to place the recess means 216 in communication with a space defined in the inverted-cup-shaped member 212, and a communication bore 218 for providing communication between the space defined in the inverted-cup-shaped member 212 and an evacuating pump (not shown) to attract the lens holder 214, hence the lens 216, into the inverted-cup-shaped member 212 by vacuum.
The lens holding device having the aforesaid construction and arrangement is capable of grinding a lens of small diameter and large thickness with high precision and helps to facilitate lense replacement and the alteration of a lens size.
The above-described prior arts, however, have not been satisfactory proposals in that they involve the following problems. The apparatus which relies on the art disclosed in Japanese Patent Laid-Open No. 121862/1986 has no mechanism for restricting radial movement of the lens 203. As a result, while grinding is being performed using a fixed abrasive grinding stone or the like, the lens 203 is pulled in the radial direction by grinding resistances, so that the axis of the lens 203 may deviate to a serious extent.
In contrast, in accordance with the art disclosed in Japanese Utility Model Publication No. 37627/1987, the radial movement of the lens 215 is restricted by the lens holder 214. However, since this lens holder 214 is attracted by vacuum into the inverted-cup-shaped member 212 via the elastic ring 213, the entire lens holder 214 is pulled in the radial direction due to grinding resistances produced during grinding, as in the art disclosed in Japanese Patent Laid-Open No. 121862/1986. This leads to the problems that the axis of the lens 215 deviates and that the elastic ring 213 is excessively compressed over its entire circumference to hinder satisfactory relaxation of elastic forces.
Because of the above-described problems, either of the aforesaid prior arts involves the serious problem that the lens may vibrate during grinding so vehemently as to disable continuation of the grinding or to deteriorate the precision of a ground surface to produce a number of imperfect products.
Conventionally, lenses of the type which can be independently ground have in most cases been subjected to nonblocking grinding (the process of grinding an object while retaining it in a holding device without the use of any adhesive). A typical example of this holder is shown in FIG. 20. In the figure, a holder 142 which constitutes the body of a holding device 141 is provided with a recess having a receiving face 143 which is covered with a receiving member 144 made of elastic material such as rubber or the like, and a lens 145 as an object to be ground is fitted into the recess with the receiving member 144 sandwiched between the lens 145 and the receiving face 143. With the aforesaid construction and arrangement, it is possible to effect nonblocking grinding without impairing the external appearance or the quality of the receiving face 146 of the lens 145 of relatively thick configuration.
In such a conventional holding device for use in a grinding apparatus, however, the overall portion of the receiving member 144 having elasticity is sandwiched and restricted in position between the holder 142 and the lens 145 owing to pressures applied to the lens 145 during grinding thereof. As a result, satisfactory relaxation of elastic force (or uniform deformation of the elastic member) is not achieved. Distorted Newton fringes, which may be produced by the influence of a variation in the strength of grinding pressure, such as an excessive increase in peripheral pressure or central pressure, are not formed in lenses of relatively thick configuration. However, if a lens to be ground has a particular configuration, for example a relatively thin configuration, variations in the strength of the grinding pressure are caused by nonuniform elastic forces. As a result, the lens is deformed and unwanted Newton fringes are formed, so that a high-precision ground surface is not obtained.